1. Field of the Invention
The present invention relates to a silica glass for photolithography, optical members including the glass, an exposure apparatus including the same, and a method for producing the glass. More particularly, it relates to a silica glass used in photolithography techniques together with light in a wavelength region of 400 nm or shorter or, more preferably, 300 nm or shorter, optical members such as lens and mirror including the glass, an exposure apparatus including the glass, and a method for producing the glass.
2. Related Background Art
In recent years, VLSI has been produced with a higher integration and a higher functionality. Particularly, in the field of logical VLSI, a larger system has been mounted on a chip, namely, system-on- chip technique has been in progress. In conjunction with such a trend, there is a demand for finer processability and higher integration on a wafer, such as that made of silicon, which constitutes a substrate for VLSI. In photolithography techniques according to which fine patterns of integrated circuits are exposed to light and transferred onto wafers such as of silicon, exposure apparatuses called stepper are used.
In the case of DRAM, as an example of VLSI, with the advance from LSI to VLSI, as its capacity gradually increases from 1 KB through 256 KB, 1 MB, 4 MB, and 16 MB to 64 MB, the processing line width required for the stepper correspondingly becomes finer from 10 xcexcm through 2 xcexcm, 1 xcexcm, 0.8 xcexcm, and 0.5 xcexcm to 0.3 xcexcm.
Accordingly, it is necessary for a projection lens of the stepper to have a high resolution and a great depth of focus. The resolution and the depth of focus are determined by the wavelength of the light used for exposure and the N.A. (numerical aperture) of the lens.
The angle of the diffracted light becomes greater as the pattern is finer, whereas the diffracted light cannot be captured when the N.A. of the lens becomes greater. Also, the angle of the diffracted light becomes smaller in the same pattern as its exposure wavelength A is shorter, thereby allowing the N.A. to remain small.
The resolution and the depth of focus are expressed as indicated by the following equations:
resolution=k1xc2x7xcex/N.A. 
depth of focus=k2xc2x7xcex/N.A.2 
wherein k1 and k2 are constants of proportionality.
In order to improve the resolution, either the N.A. is increased or xcex is shortened. However, as can be seen from the above equations, it is advantageous, in terms of the depth of focus, to shorten xcex. In view of these points of view, wavelength of light sources becomes shorter from g-line (436 nm) to i-line (365 nm) and further to KrF excimer laser beam (248 nm) and ArF excimer laser beam (193 nm).
Also, since the optical system loaded in the stepper is constituted by a combination of numerous optical members such as lenses, even when each lens sheet has a small transmission loss, such a loss is multiplied by the number of the lens sheets used, thereby decreasing the amount of light at the irradiated surface. Accordingly, it is necessary for the optical member to have a high degree of transmittance.
Therefore, in the steppers using light in a wavelength region of 400 nm or shorter, optical glass made by a specific method in view of the shortening of wavelength as well as the transmission loss due to the combination of the optical members is used. Also, in the steppers using light in a wavelength region of 300 nm or shorter, it has been proposed to use synthetic silica glass and a fluoride single crystal such as CaF, (fluorite).
As a specific method for measuring internal transmittance, for example, a method of measuring transmittance of optical glass is known from JOGIS 17-1982. Here, the internal transmittance is calculated by the following equation:                               log          ⁢                      xe2x80x83                    ⁢          τ                =                              -                                                            log                  ⁢                                      xe2x80x83                                    ⁢                  T1                                -                                  log                  ⁢                                      xe2x80x83                                    ⁢                  T2                                            Δd                                xc3x97          10                                    (        1        )            
wherein xcfx84 is internal transmittance of the glass when its thickness is 10 mm; d is difference in thickness of a sample; and T1 and T2 are spectral transmission factors of the glass having sample thickness values of 3 mm and 10 mm, respectively, including their reflection loss.
However, the inventors have found that, in the optical members composed of the conventional silica glass whose internal transmittance is defined in this manner, although a certain magnitude of the resolution is secured in terms of their specification, contrast of an image resulting therefrom may be so unfavorable that a sufficiently vivid image cannot be obtained.
Here, the contrast is defined by the following equation:                               contrast                =                                            I              ⁢                              xe2x80x83                            ⁢              max                        -                          I              ⁢                              xe2x80x83                            ⁢              min                                                          I              ⁢                              xe2x80x83                            ⁢              max                        ⁢                          xe2x80x83                        +                          I              ⁢                              xe2x80x83                            ⁢              min                                                          (        2        )            
wherein Imax is maximum value of optical intensity on a wafer surface and Imin is minimum value of the optical intensity on the wafer surface.
The object of the invention is to provide a silica glass for photolithography which can overcome the foregoing shortcomings of the prior art and can realize a sufficiently fine and vivid exposure and transfer pattern with a favorable contrast.
Accordingly, the inventors have studied, among the transmission loss factors in the silica glass (optical member) used for photolithography techniques and the like, factors for decreasing the contrast of image. As a result, it has been found that not only the optical absorption at the silica glass but also its optical scattering causes the transmission loss and that the amount of loss in light based on such optical scattering (scattering loss amount) can be sufficiently suppressed when the structure determination temperature in the silica glass containing at least a predetermined amount of OH group is reduced at least to a predetermined level. Thus, the present invention has been accomplished.
The silica glass (fused silica, quartz glass) of the present invention is used for photolithography together with light in a wavelength region of 400 nm or shorter and is characterized in that it has a structure determination temperature of 1,200 K or lower and an OH group concentration of at least 1,000 ppm.
Further, the optical member (optical component) of the present invention is an optical member used for photolithography together with light in a wavelength region of 400 nm or shorter and is characterized in that it includes the above-mentioned silica glass of the present invention.
Furthermore, the exposure apparatus (exposing device) of the present invention is an exposure apparatus which uses light in a wavelength region of 400 nm or shorter as exposure light and is characterized in that it is provided with the optical member including the above-mentioned silica glass of the present invention.
Moreover, the method for producing the silica glass in accordance with the present invention is characterized in that it comprises the steps of heating a silica glass ingot having an OH group concentration of at least 1,000 ppm to a temperature of 1,200 to 1,350 K, maintaining the ingot at that temperature for a predetermined period of time, and then cooling the ingot to a temperature of 1,000 K or lower at a temperature-lowering rate (cooling rate) of 50 K/hr or less to anneal the ingot, whereby making it possible to produce a silica glass having a structure determination temperature of 1,200 K or lower and an OH group concentration of at least 1,000 ppm.
The xe2x80x9cstructure determination temperaturexe2x80x9d herein used is a factor introduced as a parameter which expresses structural stability of silica glass and will be explained in detail below. The fluctuation in density of silica glass at room temperature, namely, structural stability is determined by density of the silica glass in the state of melt at high temperatures and density and structure of the silica glass when the density and the structure are frozen at around the glass transition point in the process of cooling. That is, thermodynamic density and structure corresponding to the temperature at which the density and structure are frozen are also retained at room temperature. The temperature when the density and structure are frozen is defined to be xe2x80x9cstructure determination temperaturexe2x80x9d in the present invention.
The structure determination temperature can be obtained in the following manner. First, a plurality of silica glass test pieces are retained at a plurality of temperatures within the range of 1073-1700 K for a period longer than the structure relaxation time (a time required for the structure of the silica glass being relaxed at that temperature) in the air in a tubular oven as shown in the accompanying FIG. 1, thereby to allow the structure of the respective test pieces to reach the structure at the retention temperature. As a result, each of the test pieces has a structure which is in the thermal equilibrium state at the retention temperature. In FIG. 1, 101 indicates a test piece, 102 indicates a silica glass tube, 103 indicates a heater, 104 indicates a thermocouple, 105 indicates a beaker, and 106 indicates liquid nitrogen.
Then, the test pieces are introduced not into water, but into liquid nitrogen in 0.2 second to quench them. If they are introduced into water, quenching is insufficient and structural relaxation occurs in the process of cooling, and the structure at the retention temperature cannot be fixed. Moreover, it can be considered that adverse effect may occur due to the reaction between water and the silica glass. In the present invention, super-quenching can be attained by introducing the test pieces into liquid nitrogen as compared with introduction into water, and by this operation, it becomes possible to fix the structure of the test pieces to the structure at the retention time. In this way, for the first time, the structure determination temperature can be allowed to coincide with the retention temperature.
The thus obtained test pieces having various structure determination temperatures (equal to the retention temperatures here) are subjected to measurement of Raman scattering, and 606 cmxe2x88x921 line intensity is obtained as a ratio to 800 cmxe2x88x921 line intensity. A graph is prepared with employing as a variable the structure determination temperature for 606 cmxe2x88x921 line intensity and this is used as a calibration curve. A structure determination temperature of a test piece of which the structure determination temperature is unknown can be inversely calculated from the measured 606 cmxe2x88x921 line intensity using the calibration curve. In the present invention, a temperature obtained in the above manner on a silica glass the structure determination temperature of which is unknown is employed as the structure determination temperature of the silica glass.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.